1. Field of the Invention
The present invention relates to a digital broadcasting receiver, and more particularly, to a vestigial sideband/quadrature amplitude modulation (VSB/QAM) receiver, which receives both a VSB signal and a QAM signal so as to be used for both ground wave and cable transmission standards, and a method thereof.
The present application is based upon Korean Patent Application No. 00-23985, which is incorporated herein by reference.
2. Description of the Related Art
Digital broadcasting services providing video and audio information for multimedia services have recently become globally commercialized. For these digital broadcasts, a bit rate large enough to transmit video and voice signals in a broadcasting channel having a predetermined bandwidth should be secured. Digital broadcasts are provided via satellite, ground wave, or cable, according to the transmission media, which use different bandwidth and modulation methods. Cable and ground wave digital broadcast signals are transmitted through the same media as existing analog broadcast signals, and thus the frequency band is limited within the UHF/VHF range. Also, due to the coexistence with the analog broadcast signals, the bandwidth is defined to be the same as the existing analog broadcast signals. Therefore, to secure a high bit rate in the limited bandwidth, multilayer level modulation methods such as quadrature amplitude modulation (QAM) or a vestigial sideband (VSB) method are adopted. Since various digital broadcasts are provided through diverse transmission media, digital broadcasting receivers (e.g., a digital TV and a digital set-top-box) should be capable of receiving both QAM signals and VSB signals. Since a VSB/QAM receiver does not need a separate VSB receiver for the VSB signals, the hardware size is reduced.
FIG. 1 illustrates an ordinary VSB/QAM receiver, which is disclosed in the BroadCOM BCM3500 Specification, “QAM/VSB CATV/HDTV Receiver” dated Jun. 2, 1999. The receiver of FIG. 1 has an analog-digital (A/D) converter 12, an automatic frequency control (AFC) 13 for restoring a carrier wave, a mixer 14, and an interpolator 15 for extracting the original symbol from the restored carrier wave. The receiver of FIG. 1 also has a matched filter 16 for filtering the restored carrier wave and a timing processor 11 for restoring carrier wave/timing and controlling clocks.
In the VSB/QAM receiver of FIG. 1, the A/D converter 12 converts an analog signal to a digital signal in accordance with a clock, which has a predetermined frequency (usually 25 MHz), generated in the timing processor 11. The AFC 13 corrects frequency and phase distortions of a signal input to the mixer 14 in accordance with a clock having a predetermined frequency generated in the timing processor 11. The mixer 14 outputs the restored carrier wave to the interpolator 15. The interpolator 15 extracts a symbol from a signal input in accordance with a symbol clock generated in the timing processor 11. The matched filter 16 filters symbols extracted in the interpolator 15. The timing processor 11 generates the symbol clock by which timing is restored and provides the symbol clock as a driving clock for the interpolator 15 and the matched filter 16. That is, a clock used in A/D conversion and carrier wave restoration has a predetermined frequency regardless of the symbol clock by which timing is restored. Here, a VSB signal is regarded as an offset QAM and the receiver has a QAM-type receiving structure as a whole.
This ordinary VSB/QAM receiver suffers from disadvantages. Since a carrier wave restoring loop is located in the front portion of the timing restoration loop, a carrier wave is restored by the A/D conversion clock and timing is restored by the symbol clock. Since in the ordinary VSB/QAM receiver the A/D conversion clock and symbol clock are not desynchronized, phase distortion can remain in a symbol and A/D conversion rate is greater than twice the ordinary symbol clock, which is disadvantageous in terms of power consumption and IC design.